This invention relates generally to magnetic disk drives, more particularly to spin valve magnetoresistive (MR) read heads, and most particularly to structures and methods for a read sensor incorporating a hybrid dual spin valve.
Magnetic disk drives are used to store and retrieve data for digital electronic apparatuses such as computers. In FIGS. 1A and 1B, a magnetic disk drive D of the prior art includes a sealed enclosure 1, a disk drive motor 2, a magnetic disk 3 supported for rotation by a spindle S1 of motor 2, an actuator 4, and an arm 5 attached to a spindle S2 of actuator 4. A suspension 6 is coupled at one end to the arm 5, and at its other end to a read/write head, or transducer 7. The transducer 7 is typically an inductive write element with a sensor read element. As the motor 2 rotates the disk 3, as indicated by the arrow R, an air bearing is formed under the transducer 7 to lift it slightly off of the surface of the disk 3. Various magnetic xe2x80x9ctracksxe2x80x9d of information can be read from the magnetic disk 3 as the actuator 4 is caused to pivot in a short arc, as indicated by the arrows P. The design and manufacture of magnetic disk drives is well known to those skilled in the art.
The most common type of sensor used in the transducer 7 is the magnetoresistive (MR) sensor. A MR sensor is used to detect magnetic field signals by means of a changing resistance in a read element. A conventional MR sensor utilizes the anisotropic magnetoresistive (AMR) effect for such detection, where the read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. When there is relative motion between the AMR sensor and a magnetic medium (such as a disk surface), a varying magnetic field from the medium causes changes in the direction of magnetization in the read element, thereby causing corresponding changes in resistance of the read element. The changes in resistance can be detected and correlated to the recorded data on the magnetic medium.
Another form of magnetoresistive effect is known as the giant magnetoresistive (GMR) effect. A GMR sensor resistance also varies in response to a changing external magnetic field, although by a different mechanism than with an AMR sensor. Sensors using the GMR effect are particularly attractive due to their greater sensitivity and higher total range in resistance than that provided by AMR sensors. One type of GMR sensor is known as a spin valve sensor. In a standard spin valve sensor there are two ferromagnetic (FM) layers separated by a layer of a non-magnetic metal such as copper. One of the ferromagnetic layers is a xe2x80x9cfree,xe2x80x9d or sensing, layer, with the orientation of its magnetization generally free to rotate in response to external fields. In contrast, the other ferromagnetic layer is a xe2x80x9cpinnedxe2x80x9d layer whose magnetization is substantially fixed, or pinned, in a particular direction. In the prior art, this pinning has typically been achieved with an exchange-coupled antiferromagnetic (AFM) layer located adjacent to the pinned layer.
More particularly, and with reference to FIG. 2, a shielded, single-element magnetoresistive head 10 includes a first shield 12, a second shield 14, and a standard spin valve 16 disposed within a gap (G) between shields 12 and 14. An air bearing surface ABS is defined by the magnetoresistive head 10. The spin valve sensor can be centered in the gap G to avoid self-biasing effects. The design and manufacture of magnetoresistive heads, such as magnetoresistive head 10, are well known to those skilled in the art.
FIG. 3, a cross-sectional view taken along line 3xe2x80x943 of FIG. 2 (i.e., from the direction of the air bearing surface ABS), illustrates the structure of the standard spin valve 16 of the prior art. The spin valve 16 includes a free layer 18, a spacer layer 20, a pinned layer 22, and an antiferromagnetic (AFM) layer 24. The spin valve 16 is supported by an insulating substrate 17 and a buffer layer 19 which can perform as a seed layer for the formation of the free layer 18 during fabrication. Ferromagnetic end regions 21, which operate as a hard bias, abut the ends of the spin valve 16 and provide stabilization of the free layer 18. Leads 25, typically made from gold or another low resistance material, allow the spin valve 16 to be joined to an electronic circuit. A capping layer 27 is provided over the AFM layer 24. A current source 29 is connected to leads 25 and provides a current IS that flows through the various layers of the standard spin valve 16. A changing magnetic field impinging upon the spin valve 16 creates a detectable change in the resistance of the spin valve 16 to the passage of an electrical current IS. A signal detection circuit 31, also in electric communication with the spin valve 16, is configured to detect changes in the resistance of the spin valve 16.
The free and pinned layers are typically made from a soft ferromagnetic material such as permalloy. As is well known to those skilled in the art, permalloy is a magnetic material nominally including 81% nickel (Ni) and 19% iron (Fe). The spacer layer 20 should be made of an electrically conductive non-magnetic material such as copper (Cu). The AFM layer 24 is used to set the magnetization orientation of the pinned layer 22, as will be discussed in greater detail below. The antiferromagnetic material of the AFM layer 24 is typically either a manganese (Mn) alloy such as iron-manganese (FeMn) or an oxide such as nickel-oxide (NiO). The AFM layer 24 prevents the magnetization of the pinned layer 22 from changing its orientation appreciably under most operating conditions, with the result being that only the orientation of the magnetization of the free layer 18 may vary in response to an external magnetic field.
FIG. 4 shows the free layer 18, the spacer layer 20, and the pinned layer 22 of standard spin valve 16. As can be seen in FIG. 4, the free layer 18 can have an actual free magnetization direction 26, while the pinned layer 22 has a pinned magnetization direction 28. The free layer 18 may have an initial free magnetization 30 in the absence of four factors, the magnetostatic coupling of the pinned layer 22, the exchange coupling through the spacer layer 20, the field generated by the sensing current IS, and any external fields. The actual free magnetization direction 26 is the sum of the initial free magnetization 30, the magnetostatic coupling of the pinned layer 22, the exchange coupling through the spacer layer 20, and the field generated by the sensing current IS. Therefore, the actual free magnetization direction 26 may be thought of as the direction that the magnetization orientation of free layer 18 will tend to return to in the absence of any external fields. As is known in the art, varying external fields, such as may be produced by a nearby magnetic medium, will preferably cause the magnetization direction of the free layer 18 to vary in response thereto, causing the electrical resistance of spin valve 16 to measurably change.
In order to achieve higher signal-to-noise ratios than those obtainable with a standard spin valve 16, dual spin valves have been developed. A typical design for a dual spin valve 40 of the prior art is shown in FIG. 5A. Dual spin valve 40 includes a first AFM layer 41, a first pinned layer 42, a first spacer 43, a free layer 44, a second spacer layer 45, a second pinned layer 46, and a second AFM layer 47. Just as with spin valve 16, the magnetization 48 of the first pinned layer 42 is substantially fixed in a particular direction by being exchange-coupled to AFM layer 41 having a fixed magnetization orientation 49. Similarly, the magnetization 50 of the second pinned layer 46 is substantially fixed in a particular direction by being exchange-coupled to AFM layer 47 having a fixed magnetization orientation 51. The dual spin valve 40 provides greater signal by approximately a factor or two.
One distinct problem with dual spin valve 40 is that the current IS, as it passes through the dual spin valve 40, creates its own magnetic field. It is well known that an electric current flowing through a conductor induces a magnetic field around the conductor. The orientation of that magnetic field is everywhere tangential to a circle around the conductor. Consequently, the current IS, as it passes through the dual spin valve 40 induces a circular magnetic field, as shown in FIG. 5B. In FIGS. 5A and 5B the current IS is shown as passing perpendicular to the plane of the drawing and oriented towards the reader as indicated by a circled dot. The induced magnetic field shown in 5B will there reinforce or enhance the magnetization 50 of pinned layer 46 while simultaneously opposing or degrading the magnetization 48 of pinned layer 42. If the direction of current IS is reversed then magnetization 50 will be opposed while magnetization 48 will be reinforced.
The reinforcement of the magnetization of the pinned layer in one spin valve is desirable, however, the opposition to the magnetization of the pinned layer in the other spin valve can create problems. Specifically, the antiferromagnetic layers 41 and 47 are sensitive to temperature, such that if heated above a certain threshold, commonly known as the blocking temperature Tb, the spins of the valence electrons within the material are no longer coupled and the magnetization of the material is lost. Typical blocking temperatures for many antiferromagnetic materials are in the range of 160xc2x0 C. to 200xc2x0 C. Therefore, as a dual spin valve 40 is heated, whether because the interior of the drive warms with use, or because of electrostatic discharges (ESD), or because of frictional heating cause by infrequent collisions between the transducer 7 and surface irregularities on the magnetic disk 3, the antiferromagnetic layers 41 and 47 may approach or exceed their blocking temperatures. If the blocking temperature is exceeded than an antiferromagnetic layer 41 or 47 loses its magnetization, then the associated pinned layer 42 or 46 will no longer be pinned and will have a magnetization orientation that is free to vary.
However, even if the blocking temperature is not exceeded, as a antiferromagnetic layer 41 or 47 approaches its blocking temperature the strength of the exchange-coupling with the adjoining pinned layer 42 or 46 weakens. If the magnetization of the pinned layer 42 or 46 is opposed by the induced magnetic field created by the current IS and the exchange-coupling strength is weak, the pinning may be overcome. Consequently, it is possible that, even though the blocking temperature of an antiferromagnetic layer 41 or 47 has not been exceeded, the adjoining pinned layer 42 or 46 may lose its fixed magnetization orientation 48 or 50. Should this occur within a spin valve that is part of a sensor, then the ability to rely on a changing resistance of the spin valve as a measure of a changing external magnetic field is lost and the sensor ceases to function.
A more complex spin valve design is illustrated in FIG. 6. FIG. 6 shows a synthetic spin valve 60 consisting of an AFM layer 61, two pinned layers 62 and 63 separated by a first spacer layer 64, a second spacer layer 65, and a free layer 66. The thickness of the first spacer layer 64 is important because for a certain range of thicknesses the pinned layers 62 and 63 on either side of the first spacer layer 64 will become antiferromagnetically coupled. As such, the magnetization 67 of pinned layer 62 will be parallel, but oppositely oriented to, the magnetization 68 of pinned layer 63. The antiferromagnetic coupling across the first spacer layer 64 is very stable and therefore difficult to overcome. The strength of this coupling provides an advantage to the synthetic spin valve 60 where an induced magnetic field from the sense current IS opposes the magnetization direction of the second pinned layer 63.
A dual spin valve may also be made from two synthetic spin valves 60 that share a common free layer 66 to take advantage of the greater sensitivity of dual spin valves over single spin valves. In order for such a dual spin valve to have this greater sensitivity it is necessary that the magnetization orientation 68 of the pinned layer 63 on one side of the free layer 66 be parallel to the magnetization orientation 68 of the pinned layer 63 on the other side of the free layer 66. The requirement that the magnetization orientations 68 of the pinned layers 63 on either side of the free layer 66 are parallel to one another imposes a restriction that the magnetization orientations 67 of the pinned layers 62 on either side of the free layer 66 are also parallel one another. Unfortunately, when a sense current is introduced, the induced magnetic field will be oriented parallel to the magnetization orientation 67 of the pinned layer 62 in one synthetic spin valve 60, while simultaneously being antiparallel to the magnetization orientation 67 of the pinned layer 62 in the other synthetic spin valve 60. Consequently, no matter which way the sense current is oriented, in one of the two synthetic spin valves 60 the magnetization of the pinned layer 67 will be opposed by the induced magnetic field of the sense current IS. As discussed above, when a sense current IS induces a magnetic field oriented antiparallel to a magnetization orientation of a pinned layer, the fixed magnetization may become unpinned as temperatures approach the blocking temperature for the material. Therefore, in a dual spin valve comprising two synthetic spin valves 60, one of the two synthetic spin valves 60 will always be subject to thermal instability resulting in a reduction in the signal strength of the device.
FIG. 7 shows yet another variation of a dual spin valve in which a standard spin valve 16 is joined with a synthetic spin valve 60. In FIG. 7 a hybrid dual spin valve 70 has a first antiferromagnetic layer 71, a first pinned layer 72, a spacer layer 73, a free layer 74, a second spacer layer 75, a second pinned layer 76 and a third pinned layer 77 separated by a third spacer layer 78, and a second antiferromagnetic layer 79. The first antiferromagnetic layer 71 has a magnetization direction 80 that pins the magnetization direction 81 of the first pinned layer 72 such that magnetization direction 81 is substantially parallel to magnetization direction 80. As with the synthetic spin valve 60, the second antiferromagnetic layer 79 has a magnetization direction 82 that pins the magnetization direction 83 of the third pinned layer 77 such that magnetization direction 83 is substantially parallel to magnetization direction 82, and also pins the magnetization direction 84 of the second pinned layer 76 such that magnetization direction 84 is substantially antiparallel to magnetization direction 82. Further, magnetization directions 83 and 84 are antiferromagnetically coupled across the third spacer layer 78.
The hybrid dual spin valve 70 is advantageous for several reasons. Firstly, the first and second pinned layers 72 and 76 on either side of free layer 74 have substantially parallel magnetization direction 81 and 84, thus a sensor incorporating such hybrid dual spin valve 70 should provide roughly twice the signal as a spin valve 16. Unlike the dual spin valve 40, or a dual synthetic spin valve, the hybrid dual spin valve 70 may be arranged such that when a current IS is introduced the induced magnetic field reinforces the magnetization 81 of the first pinned layer 72 and also reinforces the magnetization 83 of the third pinned layer 77. Thus, a hybrid dual spin valve 70 has greater thermal stability as well as improved sensitivity to external magnetic fields.
In order for a hybrid dual spin valve 70 to work well, the magnetization orientations 80 and 82 of the first and second antiferromagnetic layers 71 and 79 should be antiparallel to one another. Setting the magnetization orientations 80 and 82 of two antiferromagnetic layers 71 and 79 in antiparallel directions has not been easy to accomplish. One approach is described by Gill in U.S. Pat. No. 5,748,399 and involves the use of pulses of electric current to establish the magnetization orientations of the antiferromagnetic layers. The invention set forth in the aforementioned patent discloses that a pulse of sufficient duration and magnitude may heat the antiferromagnetic layer beyond its blocking temperature and that the magnetic field of the pulse may establish the magnetization orientation of the antiferromagnetic layer. Antiparallel orientations may then be achieved by sending electric pulses through both antiferromagnetic layers in antiparallel directions. Unfortunately, this method of resetting the magnetization orientations has several disadvantages. For example, a sensor incorporating such a system needs additional electronics to monitor the magnetization orientations of the antiferromagnetic layers and to periodically administer pulses of electric current when those magnetization orientations stray too far from their ideal directions. Further, the sensor can not operate while pulses are being applied, the electric pulses require an expenditure of additional energy, and the pulses heat the sensor by heating the antiferromagnetic layers. Such repeated heating may cause diffusion across the interfaces between the layers of the sensor causing those interfaces to degrade, ultimately lessening the sensitivity of the sensor. A further disadvantage of this technique is that the antiferromagnetic layers must be made from materials having low blocking temperatures so that the administered electric pulses can eat the layers sufficiently to allow their magnetization orientations to reset (high blocking temperature materials such as NiMn and PtMn require long-term annealing at high temperatures to set their pinning direction). However, antiferromagnetic layers made of low blocking temperature materials will inherently be more susceptible to thermal instabilities and therefore will require resetting more frequently, potentially lessening the life expectancy of the device due to the aforementioned diffusion problem.
What is desired, therefore, is a hybrid dual spin valve sensor for reading magnetic data that has antiferromagnetic layers made of high blocking temperature materials and with magnetization orientations permanently set antiparallel to one another and a method for producing the same.
The present invention provides a hybrid dual spin valve sensor for reading magnetic data that has antiferromagnetic layers with magnetization orientations permanently set antiparallel to one another, and a method for making the same.
One embodiment of the present invention provides a hybrid dual spin valve magnetoresistive read sensor. The hybrid dual spin valve sensor comprises a hybrid dual spin valve, a lead set, a current source, and a signal detection circuit. The hybrid dual spin valve comprises a 9 layer structure including, in order, a first antiferromagnetic layer, a first soft ferromagnetic layer, a first spacer layer, a free layer, a second spacer layer, a second soft ferromagnetic layer, a third spacer layer, a third soft ferromagnetic layer, and a second antiferromagnetic layer. The first antiferromagnetic layer has a magnetization orientation in a first direction and the first soft ferromagnetic layer has a magnetization orientation pinned substantially parallel to that first direction by the first antiferromagnetic layer. The second soft ferromagnetic layer has a second magnetization orientation, the third soft ferromagnetic layer has a third magnetization orientation, and the second antiferromagnetic layer has a fourth magnetization orientation. The fourth magnetization orientation is in a direction substantially antiparallel to the first direction, and consequently the second antiferromagnetic layer pins the third magnetization orientation of the third soft ferromagnetic layer also in a direction substantially antiparallel to the first direction. The third soft ferromagnetic layer and the second soft ferromagnetic layer are exchange-coupled such that the second magnetization orientation is maintained in a direction substantially parallel to the first direction. The first, second, and third spacer layers are formed of a conductive material, while the free layer is formed of a soft ferromagnetic material.
In a further embodiment, the first antiferromagnetic layer is formed of an antiferromagnetic material with a first blocking temperature and the second antiferromagnetic layer is formed of an antiferromagnetic material with a second blocking temperature. In additional embodiments the first antiferromagnetic layer is formed of IrMn and the second antiferromagnetic layer is formed of PtMn.
Another embodiment of the present invention provides a read/write head for accessing and storing data on a magnetic medium. The read/write head includes the hybrid dual spin valve magnetoresistive read sensor of the present invention along with an inductive write element. Yet another embodiment provides a magnetic data storage and retrieval system. The system comprises the read/write head of the present invention, a suspension system, and a magnetic medium. The suspension system supports the read/write head over the magnetic medium. In a further embodiment of the magnetic data storage and retrieval system the magnetic medium is rotatably supported under the read/write head and coupled to a motor for rotation about an axis.
The hybrid dual spin valve of the present invention, where each antiferromagnetic layer is made of an antiferromagnetic material with a different blocking temperature, is advantageous over the prior art because the magnetization orientations of the two antiferromagnetic layers may be conveniently and permanently set antiparallel to one another. The magnetization orientations of the two antiferromagnetic layers, once set, permanently pin the magnetization orientations of each of the soft ferromagnetic layers except for the free layer. Therefore, the electrical resistance of the hybrid dual spin valve is a simple function of the magnetization orientation of the free layer, which may vary in response to external magnetic fields. Consequently, the hybrid dual spin valve sensor, the read/write head, and the magnetic data storage and retrieval system all share the advantage of a hybrid dual spin valve having antiferromagnetic layers permanently set antiparallel to one another.
Another embodiment of the present invention includes a method for forming a hybrid dual spin valve magnetoresistive read sensor. The method includes providing a hybrid dual spin valve, fixing a magnetization orientation of a first antiferromagnetic layer, fixing a magnetization orientation of a second antiferromagnetic layer, attaching a lead set formed of a conductive material to the spin valve, attaching a current source to the lead set, and attaching a signal detection circuit to the lead set, wherein the signal detection circuit is configured to detect changes in the electrical resistance of the hybrid dual spin valve.
In a further embodiment of the method, fixing a magnetization orientation of the first antiferromagnetic layer further includes heating the hybrid dual spin valve to a first temperature, placing the spin valve within a first external magnetic field having a first orientation for a time sufficient to allow the first antiferromagnetic layer to acquire a magnetization orientation substantially parallel to the first orientation of the first external magnetic field, and cooling the spin valve within the first external magnetic field to substantially fix the magnetization orientation. In yet another embodiment of the method, fixing a magnetization orientation of the second antiferromagnetic layer further includes heating the hybrid dual spin valve to a second temperature, placing the spin valve within a second external magnetic field having a second orientation for a time sufficient to allow the second antiferromagnetic layer to acquire a magnetization orientation substantially parallel to the second orientation of the second external magnetic field, and cooling the spin valve within the second external magnetic field to substantially fix the magnetization orientation. Other embodiments of the method are directed to alternatives in which the first temperature may be either above or near the first blocking temperature, the second temperature may be either above or near the second blocking temperature, and in which the first blocking temperature may be above or below the second blocking temperature.
The method of the present invention is advantageous over the prior art in that it provides simple processes for permanently fixing the magnetization orientations of the two antiferromagnetic layers in substantially antiparallel directions. This is accomplished by fixing the magnetization orientation of the antiferromagnetic layer with the lower blocking temperature after setting the magnetization orientation of the antiferromagnetic layer with the higher blocking temperature. Because of the difference in the two blocking temperatures, the magnetization orientation of the antiferromagnetic layer with the lower blocking temperature can be fixed without altering the previously set magnetization orientation of the antiferromagnetic layer with the higher blocking temperature. The option of heating the spin valve to either above or near the blocking temperatures of the antiferromagnetic layers provides flexibility to the method by allowing the magnetization orientations of the antiferromagnetic layers to be set at either higher or lower temperatures. The choice of processing temperatures may depend, for example, on the temperature difference between the two blocking temperatures, or on other materials related considerations such as the need to keep processing temperatures below a solder eutectic temperature.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing.